Higher energy efficiency requirements for refrigerators and freezers have prompted manufacturers to investigate and develop more effective insulating materials than rigid polyurethane foam insulating materials which are currently being used. An additional motivation for finding alternatives to polyurethane foam is the growing concern over ozone depletion which will likely result in mandatory curtailment or prohibition of chlorofluoro carbons, such as CFC-11 which are commonly used as a foam blowing agent.
Improvements in foaming technology alone are not likely to result in foam insulating materials having sufficiently low thermal conductivity to preclude substantial investments in retooling to meet new energy efficiency standards. Because many currently manufactured refrigerators and freezers are near the maximum practical size that will permit passage through doorways, retooling and redesigning to meet new standards in energy efficiency with currently used insulating materials would result in reduced refrigeration capacity.
A number of alternatives are currently being explored. One method involves the use of laser-welded vacuum panels constructed from stainless steel plates. These panels have excellent insulating properties but have a number of serious disadvantages. They are heavy, expensive, not easily mass produced and conduct a significant quantity of heat at the panel ends. These panels have also had a tendency to leak at their seams whereupon their value as an insulator is practically lost.
Another alternative method of insulating refrigerators and freezers which has been investigated involves the use of an evacuated space filled with diatomaceous earth. This method suffers from many of the same disadvantages as the rigid vacuum panel, namely it would add considerable weight to refrigerators and freezers, it is much more expensive than present methods, and there have been problems with maintaining the vacuum in this type of insulating apparatus.
The use of rigid siliceous foam with vacuum bubbles has also been attempted. The difficulty with this type of insulation is that it can only be manufactured in small panels that are brittle and, consequently, are difficult to use in mass production.
It is well known that an evacuated space will serve as an effective insulator, and that decreasing absolute pressure will reduce conductive and convective heat transfer. In U.S. Pat. No. 4,027,379 issued to Cheng et al. a cryogenic vessel for holding a liquid is disclosed. The vessel is insulated by means of evacuated tubing encased in plastic foam. The patent suggests that the tubing may be made of high density polyethylene. This patent teaches that a single tube should be spirally wound around a major portion of the vessel. The tubing is closed at one end and connected to a vacuum pump at the other end to permit evacuation of the tubing to an absolute pressure of approximately one torr.
U.S. Pat. Nos. 2,969,092 and 3,007,596 described cryogenic vessels wherein the space between an inner and an outer wall is filled with insulating materials, after which the air contained therein is evacuated.
The prior art, unfortunately, fails to meet some important requirements applicable to mass production of domestic refrigerators and freezers. The most important criteria for thermal insulators used in conventional refrigerators and freezers are that they be light in weight, inexpensive, have a very low thermal conductivity, and it would be advantageous if they were adaptable to current refrigerator production techniques. When using an evacuated space as a thermal insulating barrier it is desirable to contain the vacuum within a seamless enclosure to eliminate the need for a vacuum pump as a permanent part of the insulating system and to avoid the possibility of losing the vacuum because of a small leak which could develop in a welded or fused seam.
For the foregoing reasons it is extremely desirable to develop an insulating material that employs evacuated space in a seamless, very low conductivity, light in weight, inexpensive, yet rugged form that is readily adaptable to current refrigerators and freezer production techniques.